The LTE (long term evolution) is a new generation mobile communication system brought forward by the 3GPP (the 3rd Generation Partnership Project) organization in 2004, and the system uses the OFDM (Orthogonal Frequency Division Multiplex) based wireless access technology to achieve the 100 Mbit/s downlink rate and the 50 Mbit/s uplink rate when the bandwidth is 20M. The LTE-Advance (Advanced Long Term Evolution system) is the latest generation of mobile communication system brought forward by the 3GPP organization in 2008, the system uses the wireless access technology of the carrier aggregation and is able to simultaneously run multiple pairs of uplink and downlink carriers whose maximum bandwidth is 20M, and the downlink rate is up to 1 Gbit/s, and the uplink rate is up to 500 Mbit/s.
To compensate for the pathloss of the wireless channel and reduce the LTE/LTE-A inter-cell interference, both the LTE and the LTE-Advance need to control the power of the physical uplink shared channel. The main process of the LTE uplink power control is as follows: (1) the UE (user equipment) receives a TPC (transmission power control) from the eNB (evolved NodeB); (2) the UE measures the downlink wireless channel RSRP (reference signal reception power) and calculates the Pathloss. The UE takes parameters such as the Pathloss, the uplink shared channel bandwidth, the transport block format and the TPC and so on to calculate the transmission power P; if the user equipment triggers the PHR (power headroom Report) and the transmission conditions are met, the UE also needs to send the PHR in the uplink shared channel, and its PH=Pmax−Ppusch. (3) After the eNB receives the PHR from the physical uplink shared channel, it sends the TPC to the UE through the physical downlink control channel (PDCCH) whose format is the DCI Format 0/3/3A. (4) Return to (1), that is, the UE receives the TPC.
In the aforementioned steps (2) and (3), the PHR sent by the UE to the eNB is taken as the resource budget information of the PUSCH (Physical uplink shared channel) to provide the eNB with the basis for the uplink resource allocation. If the PHR reflects that the UE has relatively large power headroom, the eNB is able to allocate relatively large number of wireless resource blocks to the UE; if the PHR reflects that the UE has no or very little power headroom, the eNB is only able to allocate few wireless resource blocks to the UE.
The PHR transmission needs two steps: the first one is triggering, and the second one is reporting. The UE triggers the PHR to report when the related timer expires, the path loss change is relatively large, or the PHR is configured/reconfigured, and after the UE acquires enough PUSCH resources, the UE reports the PHR to the eNB. The PHR is taken as the MAC (Media access control) CE (Control Element) to be multiplexed in the MAC PDU (Protocol Data Unit), and it is sent via the PUSCH.
At present, the PHR MAC CE reported by the R8/R9UE is 1 byte, and the first two bits are reserved for the system, and the latter 6 bits are the index of the specific value of the power headroom. The MAC subheader corresponding to the PHR MAC CE is one byte, and the MAC subheader uses the logical channel identification (LCID) to determine the PHR MAC CE, the LCID value is uniquely determined in the standard to distinguish from other MAC SDU, other MAC CE or Padding.
In the LTE-Advance, the UE can be configured with up to 5 uplink component carriers. Currently, the 3GPP standard meeting considers that each uplink component carrier configured for the UE should report the PHR so as to help the eNB for uplink resource allocation. The PHR comprises Type1 PHR and Type2 PHR, wherein: the Type1 PHR is the PUSCH PHR which is the same as that in the LTE, and the PHR reflects the PUSCH channel headroom; since both the PUCCH (Physical Uplink Control Channel) and PUSCH can be transmitted simultaneously in the LTE-Advance, the LTE-Advance UE also needs to send the Type2 PHR, namely PUCCH+PUSCH PHR, where PH=Pmax−Ppucch−Ppusch, and the Type2 PHR reflects the PHR of PUSCH in the case that PUCCH exists. The Type2 PHR is only sent on the UE specific UL PCC (uplink primary component carrier), while the Type1 PHR can be sent on all the UL CCs (including uplink primary component carrier and uplink secondary component carrier) configured by the UE. In addition, in order to meet the UE's overall maximum power requirements, the UE specific PHR also needs to be reported.
Therefore, in the carrier aggregation scenarios, the UE needs to report the PHR in several times, which takes up a lot of resources.